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<span id="collect2"></span><h2 class="chapter">21 <code>collect2</code></h2>

<p>GCC uses a utility called <code>collect2</code> on nearly all systems to arrange
to call various initialization functions at start time.
</p>
<p>The program <code>collect2</code> works by linking the program once and
looking through the linker output file for symbols with particular names
indicating they are constructor functions.  If it finds any, it
creates a new temporary &lsquo;<samp>.c</samp>&rsquo; file containing a table of them,
compiles it, and links the program a second time including that file.
</p>
<span id="index-_005f_005fmain"></span>
<span id="index-constructors_002c-automatic-calls"></span>
<p>The actual calls to the constructors are carried out by a subroutine
called <code>__main</code>, which is called (automatically) at the beginning
of the body of <code>main</code> (provided <code>main</code> was compiled with GNU
CC).  Calling <code>__main</code> is necessary, even when compiling C code, to
allow linking C and C++ object code together.  (If you use
<samp>-nostdlib</samp>, you get an unresolved reference to <code>__main</code>,
since it&rsquo;s defined in the standard GCC library.  Include <samp>-lgcc</samp> at
the end of your compiler command line to resolve this reference.)
</p>
<p>The program <code>collect2</code> is installed as <code>ld</code> in the directory
where the passes of the compiler are installed.  When <code>collect2</code>
needs to find the <em>real</em> <code>ld</code>, it tries the following file
names:
</p>
<ul>
<li> a hard coded linker file name, if GCC was configured with the
<samp>--with-ld</samp> option.

</li><li> <samp>real-ld</samp> in the directories listed in the compiler&rsquo;s search
directories.

</li><li> <samp>real-ld</samp> in the directories listed in the environment variable
<code>PATH</code>.

</li><li> The file specified in the <code>REAL_LD_FILE_NAME</code> configuration macro,
if specified.

</li><li> <samp>ld</samp> in the compiler&rsquo;s search directories, except that
<code>collect2</code> will not execute itself recursively.

</li><li> <samp>ld</samp> in <code>PATH</code>.
</li></ul>

<p>&ldquo;The compiler&rsquo;s search directories&rdquo; means all the directories where
<code>gcc</code> searches for passes of the compiler.  This includes
directories that you specify with <samp>-B</samp>.
</p>
<p>Cross-compilers search a little differently:
</p>
<ul>
<li> <samp>real-ld</samp> in the compiler&rsquo;s search directories.

</li><li> <samp><var>target</var>-real-ld</samp> in <code>PATH</code>.

</li><li> The file specified in the <code>REAL_LD_FILE_NAME</code> configuration macro,
if specified.

</li><li> <samp>ld</samp> in the compiler&rsquo;s search directories.

</li><li> <samp><var>target</var>-ld</samp> in <code>PATH</code>.
</li></ul>

<p><code>collect2</code> explicitly avoids running <code>ld</code> using the file name
under which <code>collect2</code> itself was invoked.  In fact, it remembers
up a list of such names&mdash;in case one copy of <code>collect2</code> finds
another copy (or version) of <code>collect2</code> installed as <code>ld</code> in a
second place in the search path.
</p>
<p><code>collect2</code> searches for the utilities <code>nm</code> and <code>strip</code>
using the same algorithm as above for <code>ld</code>.
</p>
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